Recent studies have described a 250,000 M.W. glycoprotein that is associated with proteoglycan, and seemingly expressed only by human melanoma cells. With only rare exceptions the glycoprotein can be found on the cell surface and in spent culture medium of melanoma of ocular or skin origin but not normal melanocytes both in vitro and in vivo. Preliminary evidence indicates that the glycoprotein is also expressed by gliomas and tumorgenic somatic hybrids of malignant and normal cells. Thus far, antigen, isolated from these varied sources utilizing monoclonal antibody, shows only slight variation in molecular weight by SDS-PAGE analysis and highly variable association with proteoglycan. The object of these studies is to determine if the molecular structure of this tumor associated antigen is conserved when isolated from different cellular sources. This will be accomplished with tryptic peptide mappling by reverse phase HPLC and limited N-terminal amino acid sequencing. Special emphasis will be placed on comparing glycoprotein isolated from multiple melanoma cell lines derived from the same patient. These studies should give insight into the functional significance of this human melanoma associated antigen.